Part 194. The Club Mosses - Plant Life-Histories

Part 194. The club-mosses (Class Lycopodinae) are well typified by Lycopodium (Fig. 166) which is popularly regarded as a kind of "moss" because of the general resemblance of the leaves and stems, in form and proportionate development, to the pseudo-leaves and pseudo-stems of many true mosses.

The gametophyte (Fig. 372) is bisexual and massive, as in the adder-tongues, and mostly saprophytic; and the embryo resembles that of a fern in having but a single cotyledon. Its development is essentially like that of the next type to be described.

Fig. 372.-Club-moss (Lycopodium sp., see Fig. 166.) A, gametophyte (56/1), showing archegonia (ar) and antheridia (an). B, old gametophyte (p) nursing a young sporophyte,12/1. C, antheridium (200/1) almost ready to discharge its spermatazoids. D, archegonium, cut vertically to show the egg-cells (o), the upper canal-cells dissolved into mucilage (hc), and the lower canal-cell (bc), 200/1. (Treub.)

The stem often forks but shows no secondary thickening. The leaves are unbranched, and in some species are all much alike, while in other cases the sac-leaves are smaller than the foliage leaves, are crowded into cones, and serve chiefly as protective scales for the sporangia. Each sac-leaf bears but a single spore-case on its upper surface near the base. There are no elaters.

Another large group is Selaginella (Fig. 373) the sporophytes of which often resemble those of the club-mosses so closely that they were at first included in the same genus, and many forms in cultivation are still called by florists, lycopodiums. A most significant though inconspicuous difference is that Selaginella has two kinds of spores-minute ones, called microspores,¹ which are very numerous in anther-like sacs termed microsporangia (b, Fig. 374); and macrospores ² (a) which are so large that four fill a macrosporangium. Both kinds of sporangia are borne singly on the stem just above or in the axils of upper leaves, in the same branch or cone.

¹ Mi'cro-spore - Gr: mikros, small.

² Mac'ro-spore - Gr. makros, large.

Fig. 373.-Mountain Selaginella (Selaginella helvetaci, Selaginella Family, Selaginellacece). A, sporophyte, 1/1. B, young sporophyte growing from macrospore. (Bischoff.)-Native home, Eurasian mountains.

Fig. 374.-Mountain Selaginella. Part of cone, showing a macrosporangium (a) containing three macrospores, and a microsporangium (b) discharging numerous microspores, 10/1. (Schenck.)

The spores begin to germinate while still within the sporangium. The contents of each microspore divides into several cells (Fig. 375, A-D) one of which (p) represents the vegetative part of a male gametophyte, the others constituting a male gametangium, in the center of which is formed a cluster of elongated gametes closely resembling the male gametes of a liverwort. After leaving the sporangium the microspores liberate their motile gametes by rupture of the wall. The large cell which constitutes the macrospore is rich in reserve food and begins to germinate by dividing into a number of small cells within the wall. Soon the macrospores are set free from the sporangium, and continue to germinate by forming a few archegonia on the upper side, which eventually protrudes from the ruptured spore-wall shown in Fig. 376. After fertilization, the egg-cell divides into an upper and a lower half, the lower half growing into an embryo, while the upper half develops into a peculiar organ called the suspensor (et). This by its elongation pushes the embryo, foot foremost, into the mass of vegetative cells upon which it feeds. The root and the shoot of the young embryo (Fig. 377) finally protrude from the macrospore, the foot (f) still remaining within as an organ of absorption in contact with the food supply. There are two cotyledons, which, containing chlorophyll, soon begin to make food for the plantlet, and aided by the developing leaves of the plumule, finally render the young plant self-supporting. From the upper side of each cotyledon (and often on later leaves) a flat projection (lig) termed a ligule,³ arises, which, by secreting mucilage, serves to keep the tender terminal organs from drying.

¹ Lig'ule - L. ligula, a little tongue.

Fig. 375.-Selaginellas. Germination of microspores. A-E, different views of the spore showing the prothallus-cell (p), cells of the antheridium-wall (w), and the cell producing spermatozoids (s), 640/1. In E the cell-walls have dissolved previous to discharging the spermatozoids. E, spermatozoids, 780/1. (Belajeff.)

Fig. 376.-Martin's Selaginella (Selaginella Martensii, Selaginella Family, Selaginellaceoe). Germination of macrospore 93/1, cut vertically, showing the female gametophyte protruding from the ruptured spore-wall (spm) and exposing an unfertilized archegonium (ar), but stilt enclosing two embryos (emb¹, emb²) which have been pushed down into the nutritive prothallus (pr) by their suspensors (et, et). (Pfeffer.)- Native home, Mexico; much cultivated.

Fig. 377.-Martin's Selaginella. Embryo (110/1), cut vertically to show its suspensor (et), root (w), leaves (bl, bl), ligules (tig, lig), and tip of stem (st). (Pfeffer.)

The formation of macrospores that begin to germinate while still within the sporangium, marks a most important advance in the care of offspring; for by this means not only are the chances of cross-fertilization increased, but the embryo is afforded more protection, and the young plantlet can be provided with a larger quantity of promptly available food while preparing for independent life. Just one step further is needed as we shall see, to attain the high development of parental care achieved by seed-plants. A similar differentiation of the spores and sporangia into male and female is found also in certain types of Filicinae, and in extinct Equisetinae.

As with scouring-rushes and ferns, so with the club-moss class, the modern species but feebly represent their kin of the coal age. These include giant lycopods such as Lepidodendron (Fig. 278) and Sigillaria (Fig. 277) with much-branched trunks ten meters or more in height and often a meter in thickness, bearing cones as large as those of a pine tree, and forming extensive forests.

Ths Lycopodinae, which comprise in a very few genera about 600 species, occupy an intermediate place between Filicinoe and Equisetinoe in the relative size of their leaves and in their arrangement which may be either alternate or whorled; but, while having the sac-leaves or sacs often in terminal cones, the sporangia are always solitary and never on the under side of a leaf; and there are no elaters.